1. Field of the Invention
The present invention is directed generally to monitoring the operating parameters in a steam turbine and more particularly to the monitoring of centrifugal loading of the turbine blades.
2. Description of the Prior Art
Because of the harsh environment within a steam turbine and the enormous mechanical forces which the rotating shaft and blades of the turbine experience, it has been recognized that such rotating components should be monitored for the purposes of scheduling maintenance, detecting components on the verge of failure, and the like. A variety of apparatus and methods have been developed for monitoring various components within steam turbines. For example U.S. Pat. Application Ser. No. 202,742 now U.S. Pat. No. 4,887,468 for a Non-Synchronous Turbine Blade Vibration Monitoring System, filed June 8, 1988, and U.S. Pat. Application Ser. No. 217,591 for a Turbine Blade Vibration Detection System, filed July 11, 1988, both assigned to the assignee of the present invention, are directed to monitoring the vibration of unshrouded turbine blades. U.S. Pat. No. 3,467,358 is for a Vibrator Indicator For Turboengine Rotor Blading. The system disclosed therein employes two sensors which are displaced rotation-wise relative to each other so that vibrations having an amplitude greater than a preset amplitude can be detected.
Turbine blades, however, are subjected to forces other than synchronous and nonsynchronous vibrations. For example, aerodynamic requirements mandate that large turbine blades, whether grouped or free-standing, have a twist formed between the root 12 and tip 10 as can be seen from a radial view from the tip of the blade 10 as shown in FIG. 1. That twist accommodates the increase in tangential blade velocity that exists as a monotonically increasing function of radius. The asymmetry affixed to the blade by the twist couples the axial and torsional stress in the blade. Centrifugal loading of the blade induced by rotation in the direction of arrow 36 produces forces indicated by arrows 14 in FIG. 1 which tend to untwist the blade. For example, a large free-standing blade may be subjected to sufficient centrifugal loadings to produce a three degree untwist at the blade tip at running speeds. In a lashed blade, that untwist may be small due to the constraining effect of the lashing wires. However, some degree of untwist is still experienced by the group as a whole and in the individual blades outboard of the outermost lashing wire.
The degree of untwist experienced by a turbine blade is an important factor in the design and performance of the blade. Thus, the need exists for a device that can accurately measure untwist in an economical manner which does not require substantial modification of existing steam turbines.